Medical endoprostheses or implants for the most varied applications are known in great variety from the state of the art. Implants in this sense are understood to be endovascular prostheses or other endoprostheses, for example stents, attachment elements for bones, for example screws, plates, or nails, surgical suture material, attachment elements for artificial heart valves, intestinal clamps, prostheses in the sector of hard and soft tissue, as well as anchor elements for electrodes, particularly of pacemakers or defibrillators.
Nowadays, stents, which serve for the treatment of stenoses (blood vessel occlusions), are frequently used as implants. They have a body in the form of a tubular or hollow cylindrical basic lattice, at times with perforations, which is open at both longitudinal ends. The tubular basic lattice of such an endoprosthesis is inserted into the blood vessel to be treated, and serves to support the blood vessel. Stents have particularly established themselves for the treatment of vascular diseases. By means of the use of stents, it is possible to expand occluded regions in the blood vessels, so that a lumen gain is achieved. While it is true that an optimal blood vessel cross-section that is primarily required for therapy success can be achieved by means of the use of stents or other implants, the permanent presence of such a foreign body initiates a cascade of microbiological processes that can lead to the stent slowly becoming clogged due to accretion, and, in the worst case, to vascular occlusion. Aside from this phenomenon of restenosis, permanent implants have a number of other risks: chronic inflammation, lack of growing in, late thromboses, more difficult medical reintervention, uncontrolled fatigue ruptures, etc. One approach to solving these problems consists of making the stent or other implants from a biodegradable material.
Biodegradation is understood to mean chemical, hydrolytic, enzymatic and other metabolically related decomposition processes in the living organism, which are particularly caused by the bodily fluids that come into contact with the biodegradable material of the implant, and lead to gradual dissolution of the structures of the implant that contain the biodegradable material. As a result of this process, the implant may lose some or all of its mechanical integrity at a certain point in time. The term biocorrosion is often used as a synonym for the term biodegradation. The term bioresorption includes the subsequent resorption of the decomposition products by the living organism.
Implants with an iron alloy, particularly stents that contain iron, can be produced in particularly cost-advantageous and simple manner. However, for the treatment of stenoses, for example, these implants lose their mechanical integrity, i.e. support effect, only after a comparatively long period of time, i.e. only after having stayed in the treated organism for a period of approximately two years. This means that the dwell time of implants that contain iron is too long for some applications. For other applications of the iron-containing implants, for example in orthopedics, this applies analogously for iron-based implants and for implants made of other alloys, such as some magnesium alloys, for example (e.g. WE 43).
Different mechanisms of degradation control of implants may be based, for example, on inorganic and organic protective layers or combinations of them, which resist the human corrosion milieu and the corrosion processes that occur there. Barrier layer effects are achieved, which are based on a spatial and as defect-free as possible a separation of the corrosion medium from the metallic material. These lead to the result that the degradation time is extended. Thus, the degradation protection is assured by means of protective layers having different compositions, and by means of defined geometrical distances (diffusion barriers) between the corrosion medium and the degradable metallic basic body material. Other solutions are based on alloy components of the biodegradable material of the implant body, which influence the corrosion process by means of displacement of the position in the electrochemical voltage series. Other solutions in the field of controlled degradation bring about planned breakage effects by means of applying physical (e.g. local narrowing in cross-section) and/or chemical changes in the stent surface (e.g. multilayers having locally chemically different compositions). However, in the case of iron-based implants, it is generally not possible, using the solutions mentioned above, to place the dissolution that occurs as the result of the degradation process and the crosspiece breaks that result from this into the required time window, since the stated solutions essentially bring about a lengthening in the dwell time of the material. The result is either degradation of the implant that starts too late, or an overly great variability in degradation.
Another problem in connection with coatings results from the fact that stents or other implants usually assume two states, namely a compressed state with a small diameter, and an expanded state with a greater diameter. In the compressed state, the implant can be introduced into the blood vessel to be supported, and positioned at the location to be treated. At the treatment location, the implant is then dilated, for example by means of a balloon catheter, or (when using a shape memory alloy as the implant material) transformed into the expanded state by means of heating it above a jump temperature, for example. On the basis of this change in diameter, the body of the implant is subjected to mechanical stress when this occurs. Other mechanical stresses of the implant can occur during production, or during movement of the implant in or with the blood vessel into which the implant has been inserted. In the case of the aforementioned coatings, there is therefore the disadvantage that the coating might tear during deformation of the implant (e.g. due to the formation of micro-cracks) or is even partly removed. As a result, non-specific local degradation can occur. Furthermore, the onset and speed of degradation are dependent on the size and distribution of the micro-cracks that result from the deformation, and these are difficult to control, since the micro-cracks are defects. This leads to great variation in the degradation times.